It gets sharper as I stop down - why?
It seems to be simlar with the 100-400 and the 24-70

If it's dirt & grot - how do I clean it before the Hungarian GP trip (next weekend)?

It's dust, all right! It gets sharper as you stop down because the depth of field increases. The dust itself isn't technically within that DOF, but as you stop down it gets closer to it.

I use a Giottos Rocket Blower and it works pretty well. Hold the camera with the sensor pointing at the floor and blow it out with this (or similar) tool. This way, gravity will help it not only come off the sensor, but out of the mirror box. If you just blow the dust around, the first shot will send the mirror up and down which will just re-deposit the dust to another spot on the sensor.

Check your manual - you probably got an AC adapter which you'll need to clean it. On my Fuji S2, there's a "cleaning mode" where I have to plug it in with the AC adapter, hold down two buttons and then turn it on. Then the mirror goes up, shutter opens but the sensor isn't energized. Some people cheat and just put it at "B" shutter speed but that keeps the sensor energized. Doing it that way won't help, and might just make it worse. It's really easy to do and takes me less than a minute. I give it about 10-12 blasts of air. It's easy enough that I do it pretty often. I just shot some stuff on a white background and see I need to do it again!

Since you're spending a lot of time at race tracks, you have a very good chance of this happening with all the stuff floating around in the air. Some precautions would be to change lenses as quickly as possible, and getting the new lens ready first so there's only a second or two where the body doesn't have anything on it. Point the camera sensor-down (as above) when changing lenses. But, it will happen again. Part of the fun of digital...!

Thanks.

Does anyone make blowers with filters in them?
Otherwise I just suck in dusty room air and blow it onto the sensor at high speed !

Thanks.

Does anyone make blowers with filters in them?
Otherwise I just suck in dusty room air and blow it onto the sensor at high speed !
blow it out with a vacuum cleaner hose positioned in front of the lens mount while you blow the dust off the sensor. Or go see your dentist and do what I do. I clean my cameras at the office with the dedicated dry air line (we have two air lines-one is dedicated dry air, the other is for the air/water syringe) and a suction tip positioned near the lens mount to suck out any dust as it blows out of the camera. Works really well for me-I check it with a few blank exposures of course.

Hmm I hate dentists, even the smell makes my teeth hurt :(

I'm trying the Sensor Brush at the moment, without much success I have to say.
Seems to be some smears as well after brushing - not looking good or the Grand Prix.

A hint: if you change lenses hold the camera down so that dust etc dosn't fall in while changing lenses, it's easier to clean the lens than the mirrir of sensor!

Never clean your sensor with a blower or vaccum! Leave to the pros to clean them, if you scratch the sensor or blow/suck the sensor film off your in for a major repair! I know one person who wound up spending $1500 to have the sensor replaced because they scratched it....you think dust looks bad in a shot.

I get my camera cleaned twice a year (May & August (Ok, Canon takes care of the pros at certain events and those just happen to be the ones I cover where they clean gear). Hmmm, Nikon doesn't do that.....

JS